Certain chemically resistant polymer membranes, for example, fluoropolymer membranes, are used for treating challenging fluids such as corrosive or chemically active liquids. Treatment of such fluids requires that the membranes resist chemical degradation.
However, many of these fluids, which are aqueous based, do not adequately wet the membrane surfaces which are generally of low surface energy. This inadequate wetting leads to low fluid permeability and/or high pressure drops across the membrane. This phenomenon is commonly explained by the occurrence of sites which favor the nucleation and growth of gas bubbles. Attempts have been made to improve wetting by providing a hydrophilic coating. However, many of these attempts are not satisfactory as, for example, liquid permeabilities of such membranes are still low or only marginally improved.
Further, some of the challenging liquids include high purity water, ozonated water, organic solvents, and corrosive liquids such as concentrated acids or bases. Some of these liquids may, in addition, contain an oxidizer such as a peroxide, e.g., hydrogen peroxide. These liquids tend to outgas during treatment by a membrane.
It is believed that as the liquids outgas, the gas displaces the liquid from the membrane pores. This displacement phenomenon is called “dewetting” and results in reduced effective membrane filtration area, and, consequently, reduced overall filtration efficiency. Thus, dewetting can lead to reduced permeate throughput and/or increased pressure drop.
Proposals have been made to reduce the dewetting problem. For example, membranes have been treated to rewet the dewetted membrane, e.g., by the use of a surfactant or repeated treatment with a low surface tension liquid such as isopropanol. Such approaches are not satisfactory as they involve additional costs and process steps to remove the solvent completely.
Thus, there exists a need for a wettable membrane that resists dewetting when exposed to outgassing liquids. There further exists a need for membranes that are capable of resisting degradation by challenging liquids so that the release of extractables from the membranes into the processed fluid is minimal or eliminated. There is also a need for chemically resistant membranes having a high liquid permeability.
The present invention provides for ameliorating at least some of the disadvantages of the prior art. These and other advantages of the present invention will be apparent from the description as set forth below.